This invention relates to a separator which operates effectively in weightless, or zero-gravity, conditions.
In immobile fuel cells, water formed during the combustion of oxygen and hydrogen is discharged in the form of water vapor by way of a separate hydrogen circulation. In this case, the water vapor charge of the hydrogen, similar to humidity in ambient air, is in the range of 50%-90% relative humidity. So that the circulating hydrogen is able to absorb new water vapor continuously, a separator must be installed behind the cells which reduces the absolute water vapor proportion corresponding to the charge in the hydrogen.
Since fuel cells so far have been used mainly in space operations (for example, the U.S. Apollo program, the U.S. Space Shuttle, and the European Hermes Space Plan Program), special requirements are imposed on such a gas-steam separator. These include:
secure functioning in weightless conditions and during accelerations of approximately 3 to 7 g; PA1 an energy requirement that is as low as possible; and PA1 a very low pressure loss component. PA1 1. Mechanical separation based on the centrifugal force principle, such as is described in the German patent document DE-PS 39 32 578; and PA1 2. Membrane Separation described in "Jahrbuch 1989 I der Deutschen Gesellschaft fur Luft- und Raumfahrt (DGLR)" ["Yearbook 1989 I Of the German Association for Air and Space Travel (DGLR)"], Pages 605 to 608. PA1 Separation of hydrogen and water vapor or oxygen and water vapor for immobile fuel cells in spaceships and submarines; PA1 separation of air and water vapor for the treatment of breathing air in spaceships, space suits and submarines. PA1 porous sintered metals PA1 porous ceramics PA1 porous carbon.
Heretofore, two basic physical processes have been known for the separation of a gas-water vapor mixture:
Since the latter approach is used in the process according to the invention, it will be explained in detail in the following.
In terrestrial applications, a gas - water vapor mixture can be separated in a simple manner. For example, by means of a condensation heat exchanger, the humidity-laden gas can be cooled below the dew point, so that the condensed water vapor runs off on cooling plates to the lowest point of the condenser, where the water outlet is situated. Under conditions of microgravitation, however, this principle would function only partially. That is, while the diffusion of the water vapor onto a wall cooled below the dew point also takes place fully under conditions of weightlessness, the separation of the water from the cooled wall is not ensured without additional forces, thus resulting in an undefined state in the condenser. A forced flow of the condensate film could be achieved by means of a "Slurper" (removal of the film by means of vacuum suction); however, this would result only in a preseparation, whereby an additional component would be required for the afterseparation of the gas and the water.
The simplest method of separating the condensate film is by means of suction directly through the cooled wall into a separate separation water space. For this purpose, the condenser walls must be constructed as membranes which selectively let only water pass through.
Hydrophilic porous structures which are resistant to pressure and temperature are suitable for use as membranes. The gradient for the water discharge is a pressure difference from the gas space to the separation water space. This pressure difference must be sufficiently large to ensure that all condensed water can permeate through the membrane, even during the acceleration phases (occurring hydrostatic counter pressure). For this reason, the pressure difference must be correspondingly higher for the normal operation.
The selectivity of the membrane is achieved by capillary forces on the membrane surface which, up to a certain transmembrane pressure difference (bubble point), cause the capillaries to remain filled with water, and therefore prevent a gas breakthrough. The permissible transmembrane pressure difference is a function of the theoretic capillary diameter, the shape of the pores and the wetting angle between the membrane and the water. In the above mentioned patent document, a device is provided for carrying out the process which comprises a tube-shaped membrane through which the gas mixture flows.
It is an object of the invention to provide a device for carrying out the described membrane separation process which has a construction that is as compact as possible and is reliable and relatively trouble free.
This object is achieved by means of the separator according to the invention wherein ducts, inside which the mixture of water vapor and inert gas flows, and the interior surface of which is formed by a cooled hydrophilic fine-pored membrane, extend inside one or several separation water spaces. These separation water spaces, in turn, extend inside a coolant space so that, by means of thermal conduction, the cooling of the membrane is achieved by way of the cooling of the separation water spaces. The separation water and the cooling liquid are therefore separate from one another.
In one embodiment, a single duct extends in a separation water space. In a further embodiment, several ducts are enclosed by a separation water space. In the latter, the ducts may also extend inside a large-pored supporting body, preferably made of ceramic, which is situated inside the separation water space.
Preferred applications of the invention are:
The following membrane materials may be used:
The inherent stability of the membrane is achieved by means of a thick large-pored supporting layer to which the thin fine-pored membrane layer is applied. The support and the membrane may be made of the same material and form a sintered structure, referred to as a monolithic composite, or combinations of different materials may be used for the support layer and the membrane layer. The following table indicates the materials for six self-supporting membrane structures. All have a high resistance to temperature, pressure and chemicals and therefore permit a long life of the separator.
______________________________________ Supporting Material Active Layer (Membrane) Composite of Composite of ______________________________________ Al.sub.2 O.sub.3 .alpha. Al.sub.2 O.sub.3 .gamma. ZrO.sub.2 Al.sub.2 O.sub.3 .alpha. Al.sub.2 O.sub.3 .alpha. C Composite on a (carbon fibers) carbon base C Composite on a (amorphous carbon) zirconium oxide base porous stainless steel ZrO.sub.2 (fired, gelled) ______________________________________
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.